This invention pertains to capillary supercritical fluid chromatography (SFC), and particularly to capillary SFC for simulated distillation of petroleum residues having boiling points higher than 250 degrees C.
Distillation is the most widely used separation technique in the petroleum industry. Knowledge of boiling point distribution data for crude oils and refined petroleum products is essential for process control and quality assurance. Distillation procedures, standardized by the American Society for Testing and Materials (ASTM), date back to 1926. The classical distillation procedures such as the ASTM D86, the D1160, and the D2892 require large sample sizes and are generally less precise than simulated distillation (SIMDIS) methods based on gas chromatography (GC) (See ASTM standards D87, "Annual Book of ASTM Standards", Vol. 5.01, pp. 8-27, pp. 603-615; Vol. 5.02 pp. 813-850, pp. 791-799; Vol. 5.03, pp. 442-455.
In GC-SIMDIS, the gas chromatograph can be regarded as a highly efficient microdistillation unit, while chromatography under low resolution conditions is performed. The applicability of GC-SIMDIS to the analysis of petroleum derived materials and coal-derived liquids is now well established and it has been demonstrated that GC-SIMDIS data are in excellent agreement with actual distillation methods. GC-SIMDIS methods, adopted by the ASTM since 1973, involve the use of packed columns with a nonpolar silicone gum as a stationary phase. However, the upper limit of the boiling range covered by these methods is approximately 1000 degrees Fahrenheit atmospheric equivalent boiling point (AEBP) (Hereinafter, the Fahrenheit scale is used for boiling point distribution data in accordance with ASTM methods and most other publications. Experimental conditions, as is the custom, are cited in degrees Celsius. In addition, when the term "boiling point" is used, it will mean the AEBP.).
Efforts have been undertaken to extend the scope of the GC-SIMDIS methods by employing short, thin-film capillary columns. The reason is that these columns are more favorable for the analysis of high boiling fractions because of their increased phase ratio which allows for a reduction of analysis time and column elution temperature. For instance, Trestianu et al (See HRC & CC, 1985, 8,771) found that elution temperatures on capillary columns were 100 degrees C. lower than on corresponding packed columns. Also, it has been demonstrated that capillary GC, combined with cold, on-column injection, is suitable for SIMDIS of petroleum fractions ranging from 300 to 1470 degrees F. AEBP. In this approach, however, column temperatures of up to 430 degrees C. are employed. Also by using a new type of aluminum-clad, fused silica columns for capillary GC, crude oils were chromatographed with temperatures up to 440 degrees C. These high column temperatures, however, far exceed the temperature limit &lt;&gt; 350 degrees C.) at which sample decomposition may occur and are of great concern to petroleum chemists.
The search for milder operating conditions, i.e. lower temperatures, has led to the development of SIMDIS methods other than GC, e.g. gel permeation chromatography (GPC), vacuum thermal gravimetric analysis (VTGA). In GPC, difficulties arise in the correlation of the "molecular size" distribution data to AEBP. VTGA techniques were described in the literature by Southern et al. (Anal. Chem., 1985, 57,303) and by Mondragon and Ouchi (Fuel, 1984, 63,61). The first group employed the Maxwell-Bonell equation for calculation of AEBP, while the latter used a calibration method based on actual distillation data from the ASTM D1160 method. The VTGA method performed by the latter group utilized a different approach, i.e. VTGA temperatures were calibrated with n-alkane standards in a similar fashion as boiling points of standards are related to retention times in GC-SIMDIS methods. Modification of the conventional TGA instrument for vacuum operation reduced the sample exposure to high temperatures and allowed for AEBP determinations up to approximately 1400 degrees F.
However, it is clear that none of the above approaches is completely satisfactory for high boiling point petroleum fractions due to high column temperatures.